Semiconductor structures employing strained material layers with defined impurity gradients and methods for fabricating same

ABSTRACT

Semiconductor structures and devices including strained material layers having impurity-free zones, and methods for fabricating same. Certain regions of the strained material layers are kept free of impurities that can interdiffuse from adjacent portions of the semiconductor. When impurities are present in certain regions of the strained material layers, there is degradation in device performance. By employing semiconductor structures and devices (e.g., field effect transistors or “FETs”) that have the features described, or are fabricated in accordance with the steps described, device operation is enhanced.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of, and incorporatesherein by reference, in its entirety, provisional U.S. patentapplication Ser. No. 60/324,325, filed Sep. 21, 2001.

FIELD OF THE INVENTION

The present invention relates generally to semiconductor structures anddevices and, more specifically, to semiconductor structures and fieldeffect transistors (hereinafter, “FETs”) incorporating strained materiallayers and controlled impurity diffusion gradients.

BACKGROUND OF THE INVENTION

“Virtual substrates” based on silicon (Si) and germanium (Ge) provide aplatform for new generations of VLSI devices that exhibit enhancedperformance when compared to devices fabricated on bulk Si substrates.The important component of a SiGe virtual substrate is a layer of SiGethat has been relaxed to its equilibrium lattice constant (i.e., onethat is larger than that of Si). This relaxed SiGe layer can be directlyapplied to a Si substrate (e.g., by wafer bonding or direct epitaxy) oratop a relaxed graded SiGe layer, in which the lattice constant of theSiGe material has been increased gradually over the thickness of thelayer. The SiGe virtual substrate can also incorporate buried insulatinglayers, in the manner of a silicon-on-insulator (“SOI”) wafer. Tofabricate high-performance devices on these platforms, thin strainedlayers of Si, Ge, or SiGe are grown on the relaxed SiGe virtualsubstrates. The resulting biaxial tensile or compressive strain altersthe carrier mobilities in the layers, enabling the fabrication ofhigh-speed devices, or low-power devices, or both.

A technique for fabricating strained Si wafers includes the followingsteps:

1. Providing a silicon substrate that has been edge polished;

2. Epitaxially depositing a relaxed graded SiGe buffer layer to a finalGe composition on the silicon substrate;

3. Epitaxially depositing a relaxed SiGe cap layer having a uniformcomposition on the graded SiGe buffer layer;

4. Planarizing the SiGe cap layer by, e.g., chemical mechanicalpolishing (“CMP”);

5. Epitaxially depositing a relaxed SiGe regrowth layer having a uniformcomposition on the planarized surface of the SiGe cap layer; and

6. Epitaxially depositing a strained silicon layer on the SiGe regrowthlayer.

The deposition of the relaxed graded SiGe buffer layer enablesengineering of the lattice constant of the SiGe cap layer (and thereforethe amount of strain in the strained silicon layer), while reducing theintroduction of dislocations. The lattice constant of SiGe is largerthan that of Si, and is a direct function of the amount of Ge in theSiGe alloy. Because the SiGe graded buffer layer is epitaxiallydeposited, it will initially be strained to match the in-plane latticeconstant of the underlying silicon substrate. However, above a certaincritical thickness, the SiGe graded buffer layer will relax to itsinherently larger lattice constant.

The process of relaxation occurs through the formation of misfitdislocations at the interface between two lattice-mismatched layers,e.g., a Si substrate and a SiGe epitaxial layer (epilayer). Becausedislocations cannot terminate inside a crystal, misfit dislocations havevertical dislocation segments at each end, i.e., threading dislocations,that may rise through the crystal to reach a top surface of the wafer.Both misfit and threading dislocations have stress fields associatedwith them. As explained by Eugene Fitzgerald et al., Journal of VacuumScience and Technology B, Vol. 10, No. 4, 1992, incorporated herein byreference, the stress field associated with the network of misfitdislocations affects the localized epitaxial growth rate at the surfaceof the crystal. This variation in growth rates may result in a surfacecross-hatch on lattice-mismatched, relaxed SiGe buffer layers grown onSi.

The stress field associated with misfit dislocations may also causedislocation pile-ups under certain conditions. Dislocation pile-ups area linear agglomeration of threading dislocations. Because pile-upsrepresent a high localized density of threading dislocations, they mayrender devices formed in that region unusable. Inhibiting the formationof dislocation pile-ups is, therefore, desirable.

Dislocation pile-ups are formed as follows. (See, e.g., SrikanthSamavedam et al., Journal of Applied Physics, Vol. 81, No. 7, 1997,incorporated herein by reference.) A high density of misfit dislocationsin a particular region of a crystal will result in that region having ahigh localized stress field. This stress field may have two effects.First, the stress field may present a barrier to the motion of otherthreading dislocations attempting to glide past the misfits. Thispinning or trapping of threading dislocations due to the high stressfield of other misfit dislocations is known as work hardening. Second,the high stress field may strongly reduce the local epitaxial growthrate in that region, resulting in a deeper trough in the surfacemorphology in comparison to the rest of the surface cross-hatch. Thisdeep trough may also pin threading dislocations attempting to glide pastthe region of high misfit dislocations. This cycle may perpetuate itselfand result in a linear region with a high density of trapped threadingdislocations, i.e., dislocation pile-up.

The term “MOS” (meaning “metal-oxide-semiconductor”) is here usedgenerally to refer to semiconductor devices, such as FETs, that includea conductive gate spaced at least by an insulting layer from asemiconducting channel layer. The terms “SiGe” and “Si_(1−x)Ge_(x)” arehere used interchangeably to refer to silicon-germanium alloys. The term“silicide” is here used to refer to a reaction product of a metal,silicon, and optionally other components, such as germanium. The term“silicide” is also used, less formally, to refer to the reaction productof a metal with an elemental semiconductor, a compound semiconductor oran alloy semiconductor.

One challenge to the manufacturability of MOS devices with strainedlayers is that one or more high temperature processing steps aretypically employed after the addition of the strained material. This cancause intermixing of the strained layer and underlying material. Thisintermixing is generally referred to as interdiffusion, and it can bedescribed by well-known diffusion theory (e.g., Fick's laws). Oneexample of interdiffusion is found in a FET where a strained layer isused as the channel. In this example, one or more impurities (e.g.,dopants) are implanted after addition of the strained layer. Ifimplantation is followed by a moderately high temperature step (e.g., adrive-in or anneal step), there can be rampant interdiffusion of thechannel by the implant impurity due to the presence of implant damageand excess point defects in the strained layer. A result is that theimpurity is present in the strained layer. Stated differently, theimpurity profile (i.e., a gradient describing the impurity concentrationas a function of location in the overall semiconductor or device) has anon-zero value in the strained layer. Presence of one or more impuritiesin the strained layer can, at certain concentrations, degrade overalldevice performance.

From the foregoing, it is apparent that there is still a need for a wayto produce semiconductor structures and devices that include one or morestrained layers that are not subject to the incursion of one or moreimpurity species during structure or device fabrication.

SUMMARY OF THE INVENTION

The present invention provides semiconductor structures and devices(e.g., FETs) that include one or more strained material layers that notonly improve operational performance, but also are relatively free ofinterdiffused impurities. Consequently, the resulting semiconductorstructures and devices do not exhibit the degraded performance thatresults from the presence of such impurities in the strained layers.

The invention features a semiconductor structure where at least onestrained layer is disposed on a semiconductor substrate, forming aninterface between the two. This structure is characterized by animpurity gradient that describes the concentration of one or moreimpurities (i.e., dopants) as a function of location in the structure.At the furthest part of the strained layer (i.e., a “distal zone” of thelayer away from the interface), this impurity gradient has a value thatis substantially equal to zero.

In one version of this embodiment, the invention provides a method forfabricating a semiconductor structure in a substrate. The methodincludes the step of disposing at least one strained layer on thesubstrate, forming an interface between the two. Performing at least onesubsequent processing step on the substrate, after which the impuritygradient has a value substantially equal to zero in the distal zone,follows this. The subsequent processing step is generally performedwithin a predetermined temperature range, which affects the value of theimpurity gradient, particularly in the distal zone.

In certain embodiments, the semiconductor substrate can include Si,SiGe, or any combination of these materials. It can also bemulti-layered. In this latter case, the layers can include relaxed SiGedisposed on compositionally graded SiGe. The layers can also includerelaxed SiGe disposed on Si. One or more buried insulating layers may beincluded as well.

In other embodiments, the strained layer can include Si, Ge, SiGe, orany combination of these materials. At least about fifty Angstroms ofthe furthest part of the strained layer defines a distal zone where theimpurity gradient has a value that is substantially equal to zero.

Various features of the invention are well suited to applicationsutilizing MOS transistors (e.g., FETs) that include, for example, one ormore of Si, Si_(1−x)Ge_(x) or Ge layers in or on a substrate.

In another embodiment, the invention includes a FET fabricated in asemiconductor substrate. The FET has a channel region that includes atleast one strained channel layer. The strained channel layer has adistal zone away from the substrate. The impurity gradient thatcharacterizes the substrate and the channel region has a valuesubstantially equal to zero in the distal zone.

In one version of this embodiment, the invention provides a method forfabricating a FET in a semiconductor substrate. The method includes thestep of disposing at least one strained channel layer in at least thechannel region of the FET. (The strained channel layer has a distal zoneaway from the substrate.) Performing at least one subsequent processingstep on the substrate, after which the impurity gradient has a valuesubstantially equal to zero in the distal zone, follows this. Thesubsequent processing step is generally performed within a predeterminedtemperature range, which affects the value of the impurity gradient,particularly in the distal zone.

Other aspects and advantages of the present invention will becomeapparent from the following detailed description, taken in conjunctionwith the accompanying drawings, illustrating the principles of theinvention by way of example only.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and advantages of the presentinvention, as well as the invention itself, will be more fullyunderstood from the following description of various embodiments, whenread together with the accompanying drawings, in which:

FIG. 1 is a schematic (unscaled) cross-sectional view that depicts asemiconductor structure in accordance with an embodiment of theinvention;

FIG. 2 is a schematic (unsealed) cross-sectional view that depicts a FETin accordance with an embodiment of the invention;

FIG. 3 is a flowchart depicting the steps of fabricating a FET inaccordance with an embodiment of the invention; and

FIG. 4 is a flowchart depicting the steps of fabricating a FET inaccordance with another embodiment of the invention.

DETAILED DESCRIPTION

As shown in the drawings for the purposes of illustration, the inventionmay be embodied in a semiconductor structure or device, such as, forexample, a FET, with specific structural features. A semiconductorstructure or FET according to the invention includes one or morestrained material layers that are relatively free of interdiffusedimpurities. These strained material layers are characterized by at leastone diffusion impurity gradient that has a value that is substantiallyequal to zero in a particular area of the strained layer. Consequently,the semiconductor structure or FET does not exhibit the degradedperformance that results from the presence of such impurities in certainparts of the strained layers.

In brief overview, FIG. 1 depicts a schematic (unsealed) cross-sectionalview of a semiconductor structure 100 in accordance with an embodimentof the invention. The semiconductor structure 100 includes a substrate102. The substrate 102 may be Si, SiGe, or other compounds such as, forexample, GaAs or InP. The substrate 102 may also include multiple layers122, 124, 126, 128, typically of different materials. (Although FIG. 1depicts four layers 122, 124, 126, 128, this is for illustration only. Asingle, two, or more layers are all within the scope of the invention.)

In one embodiment, the multiple layers 122, 124, 126, 128 includerelaxed SiGe disposed on compositionally graded SiGe. In anotherembodiment, the multiple layers 122, 124, 126, 128 include relaxed SiGedisposed on Si. One or more of the multiple layers 122, 124, 126, 128may also include a buried insulating layer, such as SiO₂ or Si₃N₄. Theburied insulating layer may also be doped.

In another embodiment, a relaxed, compositionally graded SiGe layer 124is disposed on a Si layer 122 (typically part of an Si wafer that may beedge polished), using any conventional deposition method (e.g., chemicalvapor deposition (“CVD”) or molecular beam epitaxy (“MBE”)), and themethod may be plasma-assisted. A further relaxed SiGe layer 126, buthaving a uniform composition, is disposed on the relaxed,compositionally graded SiGe layer 124. The relaxed, uniform SiGe layer126 is then planarized, typically by CMP. A relaxed SiGe regrowth layer128 is then disposed on the relaxed, uniform SiGe layer 126.

One or more strained layers 104 are disposed on the substrate 102.Between the substrate 102 and the strained layer 104 is an interface106. Located away from the interface 106 is the distal zone 108 of thestrained layer 104.

In various embodiments, the strained layer 104 includes one or morelayers of Si, Ge, or SiGe. The “strain” in the strained layer 104 may becompressive or tensile, and it may be induced by lattice mismatch withrespect to an adjacent layer, as described above, or mechanically. Forexample, strain may be induced by the deposition of overlayers, such asSi₃N₄. Another way is to create underlying voids by, for example,implantation of one or more gases followed by annealing. Both of theseapproaches induce strain in the underlying substrate 102, in turncausing strain in the strained layer 104.

The substrate 102, strained layer 104, and interface 106 arecharacterized, at least in part, by an impurity gradient 110A, 110B(collectively, 110). The impurity gradient 110 describes theconcentration of the impurity species as a function of location acrossthe substrate 102, strained layer 104, and interface 106. The impuritygradient 110 may be determined by solving Fick's differential equations,which describe the transport of matter: $\begin{matrix}{J = {{- D}\frac{\partial N}{\partial x}}} & \text{(Equation~~1)}\end{matrix}$

$\begin{matrix}{\frac{\partial N}{\partial t} = {D\frac{\partial^{2}N}{\partial x^{2}}}} & \text{(Equation~~2)}\end{matrix}$

In equations (1) and (2), “J” is the impurity flux, “D” is the diffusioncoefficient, and “N” is the impurity concentration. Equation (1)describes the rate of the permeation of the diffusing species throughunit cross sectional area of the medium under conditions of steady stateflow. Equation (2) specifies the rate of accumulation of the diffusingspecies at different points in the medium as a function of time, andapplies to transient processes. In the general case, equations (1) and(2) are vector-tensor relationships that describe these phenomena inthree dimensions. In some cases, equations (1) and (2) may be simplifiedto one dimension.

The steady state solution to equation (1), which is not detailed herein,is a function of the Gaussian error function: $\begin{matrix}{{{erf}(y)} = {\frac{2}{\sqrt{\pi}}{\int_{0}^{y}{^{- z^{2}}{z}}}}} & \text{(Equation~~3)}\end{matrix}$

An example solution is shown in FIG. 1 as the impurity gradient 110.Axis 112 represents the impurity concentration, typically in units ofcm⁻³. Axis 114 corresponds to the location in the semiconductorstructure 100. Axis 114 is aligned with the semiconductor structure 100to illustrate a typical impurity profile, meaning that the impurityconcentration at any point in the semiconductor structure 100 can beascertained as a function of location. Except as described below, thedepicted shape of the impurity gradient 110 is not intended to belimiting. For example, impurity gradient 110A may describe a profile ofa p-type (e.g., boron) or n-type (e.g., phosphorous or arsenic) dopantintroduced in the substrate 102. On the other hand, impurity gradient110B may, for example, describe a substantially constant concentrationof Ge, or Si, or both, in the substrate 102 that takes on a desiredvalue (e.g., a reduced value) in the strained layer 104. Stateddifferently, the impurity gradient 110 may describe the concentration ofany species in the substrate 102, including the substrate speciesitself, at any point in the semiconductor structure 100.

Boundary 116 represents the interface between the substrate 102 and thestrained layer 104. Boundary 118 depicts the start of the distal zone108 of the strained layer 104. Boundary 120 corresponds to the edge ofthe strained layer 104. Of note are the locations where the boundaries116, 118, 120 intersect the axis 114 and the impurity gradient 110. Inparticular, the impurity gradient 110 has a value substantially equal tozero in the distal zone 108. This is depicted by the impurity gradient110 approaching the axis 114 at boundary 118, and remaining there, or atzero, or at another value substantially equal to zero, between boundary118 and 120. Of course, the impurity gradient 110 can also have a valuesubstantially equal to zero before reaching the boundary 118. In anycase, one embodiment of the invention features a distal zone 108 thatincludes at least about fifty Angstroms of the furthest part of thestrained layer 104. That is, the distal zone 108 is at least about fiftyAngstroms thick.

In another embodiment depicted schematically (i.e., unsealed) in FIG. 2,a FET 200 is fabricated in a similar semiconductor structure. The FET200 includes a semiconductor substrate 202, which may be Si, SiGe, orother compounds such as, for example, GaAs or InP. The substrate 202 canbe multi-layered, and it can include relaxed SiGe disposed oncompositionally graded SiGe, or relaxed SiGe disposed on Si. Thesubstrate 202 may also include a buried insulating layer, such as SiO₂or Si₃N₄. The buried insulating layer may also be doped.

Disposed on the substrate 202 is an isolation well 204, typicallyincluding an oxide. Within the isolation well 204 are isolation trenches206. A source region 208 and a drain region 212 are typically formed byion implantation. A FET channel 210 is formed from one or more strainedlayers. The strained layers can include one or more layers of Si, Ge, orSiGe. The “strain” in the strained layers may be compressive or tensile,and it may be induced as described above. The furthest part of thechannel 210 is located away from the substrate 202. This furthest partforms the distal zone of the channel 210.

Disposed on at least part of the channel 210 is a gate dielectric 214,such as, for example, SiO₂,Si₃N₄, or any other material with adielectric constant greater than that of SiO₂ (e.g., HfO₂, HfSiON). Thegate dielectric 214 is typically twelve to one hundred Angstroms thick,and it can include a stacked structure (e.g., thin SiO₂ capped withanother material having a high dielectric constant).

Disposed on the gate dielectric 214 is the gate electrode 216. The gateelectrode 216 material can include doped or undoped polysilicon, dopedor undoped poly-SiGe, or metal. Disposed about the gate electrode 216are the transistor spacers 218. The transistor spacers 218 are typicallyformed by depositing a dielectric material, which may be the samematerial as the gate dielectric 214, followed by anisotropic etching.

The impurity gradient 110 also characterizes the channel 210 and thesubstrate 202, as well as the isolation well 204. This is shown in FIG.2 in an expanded view that, for clarity, differs in scale compared tothe remainder of (unscaled) FIG. 2. The distal zone of the channel 210corresponds to that portion of the impurity gradient 110 betweenboundaries 118, 120 (expanded for clarity). Within the distal zone ofthe channel 210, the impurity gradient 110 has a value substantiallyequal to zero. As discussed above, the depicted shape of the impuritygradient 110 is not intended to be limiting, and the impurity gradient110 can also have a value substantially equal to zero before reachingthe boundary 118. One embodiment of the invention features a distal zone108 that includes at least about fifty Angstroms of the furthest part ofthe channel 210. That is, the distal zone is at least about fiftyAngstroms thick.

One version of an embodiment of the invention provides a method forfabricating a FET in a semiconductor substrate. The method includes thestep of disposing one or more strained channel layers in the FET channelregion. The channel layer has a distal zone away from the substrate. Thedistal zone includes at least about fifty Angstroms of the furthest partof the channel region. An impurity gradient characterizes at least thesubstrate and the strained layers.

Next, one or more subsequent processing steps are performed on thesubstrate. After these subsequent processing steps are performed, theimpurity gradient has a value that is substantially equal to zero in thedistal zone. Since the impurity gradient can be influenced bytemperature, the subsequent processing steps are typically performedwithin a predetermined temperature range that is chosen to ensure thatthe impurity gradient has a desired value, particularly in the distalzone.

FIG. 3 depicts a method 300 for fabricating the FET in accordance withan embodiment of the invention. This method includes the step ofproviding a substrate, typically planarized, and without strained layers(step 302). The substrate can include relaxed SiGe on a compositionallygraded SiGe layer, relaxed SiGe on a Si substrate, relaxed SiGe on Si,or other compounds such as GaAs or InP. The substrate can also contain aburied insulating layer.

Next, initial VLSI processing steps are performed such as, for example,surface cleaning, sacrificial oxidation, deep well drive-in, andisolation processes like shallow trench isolation with liner oxidationor LOCOS (step 304). Any number of these steps may include hightemperatures or surface material consumption. Features defined duringstep 304 can include deep isolation wells and trench etch-refillisolation structures. Typically, these isolation trenches will berefilled with SiO₂ or another insulating material, examples of which aredescribed above.

Next, the channel region is doped by techniques such as shallow ionimplantation or outdiffusion from a solid source (step 306). Forexample, a dopant source from glass such as BSG or PSG may be deposited(step 308), followed by a high temperature step to outdiffuse dopantsfrom the glass (step 310). The glass can then be etched away, leaving asharp dopant spike in the near-surface region of the wafer (step 312).This dopant spike may be used to prevent short-channel effects in deeplyscaled surface channel FETs, or as a supply layer for a buried channelFET that would typically operate in depletion mode. The subsequentlydeposited channel layers can then be undoped, leading to lessmobility-limiting scattering in the channel of the device and improvingits performance. Likewise, this shallow doping may be accomplished viadiffusion from a gas source (e.g., rapid vapor phase doping or gasimmersion laser doping) (step 314) or from a plasma source as in plasmaimmersion ion implantation doping (step 316).

Next, deposit one or more strained channel layers, preferably by a CVDprocess (step 318). The channel may be Si, Ge, SiGe, or a combination ofmultiple layers of Si, Ge, or SiGe. Above the device isolation trenchesor regions, the deposited channel material typically will bepolycrystalline. Alternatively, the device channels may be depositedselectively, i.e., only in the device active area and not on top of theisolation regions. Typically, the remaining steps in the transistorfabrication sequence will involve lower thermal budgets and little or nosurface material consumption.

Next, the transistor fabrication sequence is continued with the growthor deposition of a gate dielectric (step 320) and the deposition of agate electrode material (step 322). Examples of gate electrode materialinclude doped or undoped polysilicon, doped or undoped poly-SiGe, ormetal. This material stack is then etched (step 324), forming the gateof the transistor. Typically, this etch removes the gate electrodematerial by a process such as reactive ion etching (“RIE”) and stops onthe gate dielectric, which is then generally removed by wet etching.After this, the deposited channel material typically is still present.

Next, the transistor spacers are formed by the traditional process ofdielectric material deposition and anisotropic etching (step 326). Step326 may be preceded by extension implantation, or removal of the channelmaterial in the regions not below the gate, or both. If the channelmaterial is not removed before spacer material deposition, the spaceretch may be tailored to remove the excess channel material in theregions not below the gate. Failure to remove the excess channelmaterial above the isolation regions can result in device leakage paths.

Next, the source and drain regions are fabricated, typically by ionimplantation (step 328). Further steps to complete the devicefabrication can include salicidation (step 330) and metallization (step332).

FIG. 4 depicts another method 400 for fabricating the FET in accordancewith an embodiment of the invention. This method includes creating thechannel at a different point in the fabrication process, and starts withperforming the traditional front-end VLSI processing steps, such as, forexample, well formation, isolation, gate stack deposition anddefinition, spacer formation, source-drain implant, silicidation (step402). In place of a gate electrode, fabricate a “dummy gate” (step 404).This dummy gate is etched and replaced in subsequent processing steps.The dummy gate may include an insulating material such as Si₃N₄ (or anyof the other dielectric materials discussed), or a conducting materialsuch as polysilicon, poly-Ge, or metal. In contrast to a typical MOSFETprocess where the gate is separated from the semiconductor substrate bya gate dielectric, the dummy gate is separated from the substrate by anetch-stop layer. The etch-stop layer can be of SiO₂, either thermallygrown or deposited.

Next, a dielectric layer is deposited (e.g., by a CVD process) (step406) and planarized (step 408) by, for example, CMP. This “planarizationlayer” is typically a different material then the dummy gate.

Next, the dummy gate is removed by a selective etching process (step410). The etch-stop layer protects the substrate from this etchingprocess. A wet or dry etch then removes the etch-stop layer.

An example configuration includes a polysilicon dummy gate, an SiO₂etch-stop layer, Si₃N₄ spacers, and an SiO₂ planarization layer. Thisconfiguration allows selective removal of the dummy gate with an etchantsuch as heated tetramethylammonium hydroxide (“TMAH”), thereby leavingthe SiO₂ and Si₃N₄ intact. The etch-stop is subsequently removed by awet or dry etch (e.g., by HF).

Next, one or more strained channel layers is deposited, typically by aCVD process (step 412). The channel layers may be Si, Ge, SiGe, or acombination of multiple layers of Si, Ge, or SiGe. The gate dielectricis then thermally grown or deposited (by CVD or sputtering, for example)(step 416). This is followed by deposition of the gate electrodematerial (step 418), which can include doped or undoped polysilicon,doped or undoped poly-SiGe, or metal.

Next, the gate electrode is defined (step 420). This can be byphotomasking and etching (step 422) of the gate electrode material. Thismay also be done by a CMP step (step 424), where the gate electrodematerial above the planarization layer is removed.

Using this method, a silicide is generally formed on the source anddrain regions before the deposition of the planarization layer. In thiscase, all subsequent processing steps are typically limited to atemperature that the silicide can withstand without degradation. Onealternative is to form the silicide at the end of the process. In thiscase, the planarization layer may be removed by a selective wet or dryetch which leaves the gate electrode material and the spacers intact.This is followed by a traditional silicide process, e.g., metaldeposition and thermally activated silicide formation on the source anddrain regions (and also on the gate electrode material, if the latter ispolysilicon), followed by a wet etch strip of unreacted metal. Furthersteps to complete the device fabrication can include inter-layerdielectric deposition and metallization. Note that if the step offorming the gate dielectric is omitted, a metal gate electrode may bedeposited directly on the channel, resulting in the fabrication of aself-aligned HEMT (or MESFET) structure.

From the foregoing, it will be appreciated that the semiconductorstructures and devices provided by the invention afford a simple andeffective way to minimize or eliminate the impurities in certain partsof strained material layers used therein. The problem of degraded deviceperformance that results from the presence of such impurities is largelyeliminated.

One skilled in the art will realize the invention may be embodied inother specific forms without departing from the spirit or essentialcharacteristics thereof. The foregoing embodiments are therefore to beconsidered in all respects illustrative rather than limiting of theinvention described herein. Scope of the invention is thus indicated bythe appended claims, rather than by the foregoing description, and allchanges which come within the meaning and range of equivalency of theclaims are therefore intended to be embraced therein.

What is claimed is:
 1. A semiconductor structure comprising: a substrate; and at least one strained layer disposed on the substrate, thereby defining an interface therebetween, the at least one strained layer having a distal zone away from the interface; wherein the substrate, the interface, and the at least one strained layer are characterized at least in part by an impurity gradient having a value substantially equal to zero in the distal zone.
 2. The semiconductor structure of claim 1 wherein the substrate comprises Si.
 3. The semiconductor structure of claim 1 wherein the substrate comprises SiGe.
 4. The semiconductor structure of claim 1 wherein the substrate comprises a p-type dopant.
 5. The semiconductor structure of claim 1 wherein the substrate comprises an n-type dopant.
 6. The semiconductor structure of claim 1 wherein the substrate comprises a plurality of layers.
 7. The semiconductor structure of claim 6 wherein the plurality of layers comprises relaxed SiGe disposed on compositionally graded SiGe.
 8. The semiconductor structure of claim 6 wherein the plurality of layers comprises relaxed SiGe disposed on Si.
 9. The semiconductor structure of claim 6 wherein the plurality of layers comprises a buried insulating layer.
 10. The semiconductor structure of claim 1 wherein the at least one strained layer comprises Si.
 11. The semiconductor structure of claim 1 wherein the at least one strained layer comprises Ge.
 12. The semiconductor structure of claim 1 wherein the at least one strained layer comprises SiGe.
 13. The semiconductor structure of claim 1 wherein the distal zone comprises at least about fifty Angstroms of the at least one strained layer.
 14. The semiconductor structure of claim 10 wherein the impurity gradient describes at least the concentration of Ge.
 15. The semiconductor structure of claim 11 wherein the impurity gradient describes at least the concentration of Si.
 16. A FET fabricated in a semiconductor substrate, the FET comprising a channel region including at least one strained channel layer, the at least one strained channel layer having a distal zone away from the substrate, wherein the substrate and the channel region are characterized at least in part by an impurity gradient having a value substantially equal to zero in the distal zone.
 17. The FET of claim 16 wherein the substrate comprises Si.
 18. The FET of claim 16 wherein the substrate comprises SiGe.
 19. The FET of claim 16 wherein the substrate comprises a p-type dopant.
 20. The FET of claim 16 wherein the substrate comprises an n-type dopant.
 21. The FET of claim 16 wherein the substrate comprises a plurality of layers.
 22. The FET of claim 21 wherein the plurality of layers comprises relaxed SiGe disposed on compositionally graded SiGe.
 23. The FET of claim 21 wherein the plurality of layers comprises relaxed SiGe disposed on Si.
 24. The FET of claim 21 wherein the plurality of layers comprises a buried insulating layer.
 25. The FET of claim 16 wherein the at least one strained channel layer comprises Si.
 26. The FET of claim 16 wherein the at least one strained channel layer comprises Ge.
 27. The FET of claim 16 wherein the at least one strained channel layer comprises SiGe.
 28. The FET of claim 16 wherein the distal zone comprises at least about fifty Angstroms of the at least one strained channel layer.
 29. The FET of claim 25 wherein the impurity gradient describes at least the concentration of Ge.
 30. The FET of claim 26 wherein the impurity gradient describes at least the concentration of Si.
 31. A method for fabricating a semiconductor structure in a substrate, the method comprising the steps of: disposing at least one strained layer on the substrate thereby defining an interface therebetween, the at least one strained layer having a distal zone away from the interface; and performing at least one subsequent processing step on the substrate after which an impurity gradient characterizing, at least in part, the substrate, the interface, and the at least one strained layer has a value substantially equal to zero in the distal zone.
 32. The method of claim 31 wherein the substrate comprises Si.
 33. The method of claim 31 wherein the substrate comprises SiGe.
 34. The method of claim 31 wherein the substrate comprises a p-type dopant.
 35. The method of claim 31 wherein the substrate comprises an n-type dopant.
 36. The method of claim 31 wherein the substrate comprises a plurality of layers.
 37. The method of claim 36 wherein the plurality of layers comprises relaxed SiGe disposed on compositionally graded SiGe.
 38. The method of claim 36 wherein the plurality of layers comprises relaxed SiGe disposed on Si.
 39. The method of claim 36 wherein the plurality of layers comprises a buried insulating layer.
 40. The method of claim 31 wherein the at least one strained layer comprises Si.
 41. The method of claim 31 wherein the at least one strained layer comprises Ge.
 42. The method of claim 31 wherein the at least one strained layer comprises SiGe.
 43. The method of claim 31 wherein the distal zone comprises at least about fifty Angstroms of the at least one strained layer.
 44. The method of claim 31 wherein the at least one subsequent processing step is performed within a predetermined temperature range.
 45. The method of claim 40 wherein the impurity gradient describes at least the concentration of Ge.
 46. The method of claim 41 wherein the impurity gradient describes at least the concentration of Si.
 47. A method for fabricating a FET in a semiconductor substrate, the FET comprising a channel region, the method comprising the steps of: disposing at least one strained channel layer in at least the channel region, the at least one strained channel layer having a distal zone away from the substrate; and performing at least one subsequent processing step on the substrate after which an impurity gradient characterizing, at least in part, the substrate and the at least one strained layer has a value substantially equal to zero in the distal zone.
 48. The method of claim 47 wherein the substrate comprises Si.
 49. The method of claim 47 wherein the substrate comprises SiGe.
 50. The method of claim 47 wherein the substrate comprises a p-type dopant.
 51. The method of claim 47 wherein the substrate comprises an p-type dopant.
 52. The method of claim 47 wherein the substrate comprises a plurality of layers.
 53. The method of claim 52 wherein the plurality of layers comprises relaxed SiGe disposed on compositionally graded SiGe.
 54. The method of claim 52 wherein the plurality of layers comprises relaxed SiGe disposed on Si.
 55. The method of claim 52 wherein the plurality of layers comprises a buried insulating layer.
 56. The method of claim 47 wherein the at least one strained layer comprises Si.
 57. The method of claim 47 wherein the at least one strained layer comprises Ge.
 58. The method of claim 47 wherein the at least one strained layer comprises SiGe.
 59. The method of claim 47 wherein the distal zone comprises at least about fifty Angstroms of the at least one strained layer.
 60. The method of claim 47 wherein the at least one subsequent processing step is performed within a predetermined temperature range.
 61. The method of claim 56 wherein the impurity gradient describes at least the concentration of Ge.
 62. The method of claim 57 wherein the impurity gradient describes at least the concentration of Si. 